306 THE ILLINOIS GLACIAL LOBE. 



covering a few square miles in which a sharp knob-and-basin topography is 

 developed, around which are gently undulating tracts, such as occur on the 

 east slope. There are also extensive marshes west of this crest, and the 

 drainage is generally less perfect than on the east slopes. Lakes abound in 

 Lake County on both the west and the east slopes. They range in size 

 from an area of several square miles down to an acre or less.' They are 

 usually bordered wholly or in part by knolls and ridges which rise 10 to 

 50 feet above their surface. In some cases extensive marshes border one 

 or more sides of the lakes. These lakes, with their bordering hills dotted 

 with groves, add greatly to the beauty of the scenery. The lakes and also 

 basins become rare southward in Cook County. The few which occur are 

 very small and shallow. 



THICKNESS OF DRIFT. 



Records of several deep borings were obtained which give an average 

 thickness of about 200 feet of drift. Six borings failed to reach rock at an 

 average depth of about 250 feet, one of them being 315 feet. As these 

 borings are widely distributed, it seems probable that the general level of 

 the rock surface is fully 200 feet below the drift surface. The underlying 

 rock is limestone and presents a very uneven surface. The well borings 

 indicate that valleys had been cut to depths of 200 feet or more prior to the 

 drift deposition. Hence borings in the lines of these valleys, when located 

 on high parts of the moraine, probably would encounter fully 400 feet of 

 drift. The thickness of each of the several drift sheets here present has not 

 been worked out so fully as in some other localities, but the drift of Wisconsin 

 age apparently averages not less than 100 feet in depth, and may average 

 150 feet. 



STRUCTURE OF THE DRIFT. 



The upper portion of the drift to a depth of 100 to 150 feet consists 

 mainly of till. The till is usually oxidized at surface to a depth of 10 or 

 15 feet, beneath which it presents a blue-gray color. It is also generally 

 soft and fresh, and seems to be referable to the Wisconsin, though it may 

 also include the Iowan. At greater depths than 100 to 150 feet borings 

 encounter either a hard gray till tinged with brown, probably Illinoian, or 

 beds of sand and gravel, or an alternation of till with sand and gravel. 

 Wells near Lake Zurich have in several instances encountered thick beds of 





